The original story is described here as "The 1902 invasion" but that year is never mentioned in the book. The first chapter describes the gun being cast on the 2nd of August 1894 before saying "the storm burst upon us six years ago now" and describing the shots which started on the 12th. I think that's been taken to mean the shots were fired 6 years after the gun was cast but surely it means the story is being written 6 years later. If that's the case then the invasion would have landed around June 1895.
Would be very interested to hear other people's thoughts.